In recent years, lithium secondary batteries are widely used as power supplies for electronic equipment such as mobile telephones and laptop computers, or for electric cars and electric power storage. Particularly recently, there is a rapidly increasing demand for batteries having high capacities, high power outputs and high energy densities, which can be mounted in hybrid cars or electric cars.
A lithium secondary battery is composed mainly of a positive electrode and a negative electrode, respectively containing a material capable of storing and releasing lithium, and a non-aqueous electrolyte solution containing a lithium salt and a non-aqueous solvent.
As a positive electrode active material that is used in the positive electrode, for example, a lithium metal oxide such as LiCoO2, LiMnO2, LiNiO2, or LiFePO4 is used.
Furthermore, as the non-aqueous electrolyte solution, a solution prepared by mixing a Li electrolyte such as LiPF6, LiBF4, LiN(SO2CF3)2 or LiN(SO2CF2CF3)2, with ethylene carbonate, propylene carbonate, or a solvent mixture of carbonates such as ethylene carbonate and methyl carbonate (non-aqueous solvent), is used.
On the other hand, as a negative electrode active material that is used in the negative electrode, lithium metal, a metal compound capable of storing and releasing lithium (a simple metal, an oxide, an alloy with lithium, or the like), or a carbon material is known. Particularly, lithium secondary batteries employing cokes, artificial graphite and natural graphite, all of which are capable of storing and releasing lithium, have been put to practical use.
Among the battery performances, particularly in connection with lithium secondary batteries for automobile use, high power output is demanded. Therefore, it is desirable to control the resistance of batteries to a low level over various conditions.
Known as one of the factors by which the resistance of batteries increases is a film formed from the decomposition product of a solvent or from an inorganic salt, which is formed on the surface of the negative electrode. Generally, it is known that at the surface of the negative electrode, a reductive decomposition reaction of the electrolyte solution occurs under charging conditions, since lithium metal is present in the negative electrode active material. In the case in which such reductive decomposition continuously occurs, the resistance of the battery increases, the charge-discharge efficiency decreases, and the energy density of the battery is decreased. In order to overcome these problems, attempts have been made to add various compounds to electrolyte solutions.
As such attempts, attempts have been made to improve the battery resistance by incorporating various sulfonic acid ester compounds as the additives (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2000-3724, 2000-133304, WO 2005/057713, and JP-A No. 2009-054287).